The need for effective air purification is ever increasing. The need is evident in providing a healthy home environment and comfortable conditions in offices and vehicles, and also in creating clean room environments for advanced production and research.
In existing systems for filtering of air, particularly in larger buildings, the air purification is often combined with ventilation systems. The air that is forced into the building is typically made to pass through one or more filters with the purpose of reducing the amount of particles in the air. The air filters are normally divided into three classes: coarse filter, high efficiency filter and micro filter (or High Efficiency Particulate Air (HEPA) filter). Coarse and high efficiency filters are often used in combination and can be made to be effective in blocking particles of a size larger than 1 μm. However, a significant portion of the air pollution comprises of particles with sizes below 1 μm, and among these are substances that has been shown to be of importance regarding health aspects. Furthermore, in clean rooms, the requirements concerning the size of particles and their concentrations are significantly harder than what can be provided with the combination of coarse and high efficiency filters only. To effectively filter out particles smaller than 1 μm, commonly used techniques comprises the uses of one or more micro filters. The micro filters are comprised of materials of extremely fine fibers giving very large active area of the filter. The term “density” is a measure of the level of filtering. It is possibly to obtain high degrees of filtering using micro filters. However, these filters are typically expensive and due to their high density they often have a short lifetime. Hence, the micro filters need to be frequently replaced, which result in high maintenance costs. Additionally, due to their high density, the micro filters cause a large pressure drop, not at least than the filters are starting to get clogged. This results in a lower air flow and a deterioration of the ventilation if the filters are not frequently exchanged. Alternatively, the fans of the ventilation system need to work harder, which gives higher cost of operation and often an increase in noise. Accordingly, although technically possible, it is costly and cumbersome to achieve and maintain a high degree of air purification using micro filters and at the same time provide a sufficient ventilation.
The above described filters are functioning through their density only. Other principles of purging air is known in the art. Among the known principles are the use of electrostatic filters. These filters have in combination to having a high density, also been provided with a electrostatic charge in order to attract charged air particles (see for example www.camfil.se). The material in the electrostatic filters are good insulators, for example polymers, and the filter gets the electrostatic charge during the manufacturing or mechanical machining of the filter, for example.
Electrostatic filters can be effective, but their electrostatic charge is often relatively quickly neutralized by particles of opposite charge which are caught in the filter, which leads to a deterioration in efficiency of the filters. Further, the filters are typically sensitive to moisture in the air, which will lower the electrostatic charge of the filters. In practice, then used in realistic conditions, the electrostatic filters typically have lost a significant part of their capacity already after a couple of weeks.
Electric charge is used also in air purification using so-called ionizers. Ionizers transfers charge in the form of electrons to airborne particles and molecules. These negatively charged particles can more easily agglomerate to larger particles which can be made to attract to a positive part of the ionizer. Alternatively high efficiency or micro filters are used to collect the agglomerated particles. Similarly, positive airborne particles can be made. The efficiency of the filters can be increased further if the filters are given an electrostatic charge. If high voltage ion emitters are utilized, actual ionization of the surrounding gas (air) occur due to so called corona discharge. The so-created ions collide with and stick to the airborne particles. Depending on the potential of the emitter, positive or negative ions may be created. Such filter and filtering systems are commercially available from Transjonic AB, for example.
Transjonic discloses an arrangement for air purification intended to supplement or replace a conventional air purification in a ventilation system. One or more ion emitters are placed in an air duct for the incoming air stream, close to a filter package. An improved air purification is achieved both due to that small particles agglomerates more easily to larger entities which can be caught by a high efficiency filter, for example, and due to electrostatic effects in the filters. However, the electrostatic filters have the same shortcomings as described above, i.e. the electrostatic filters lose their charge during operation and hence, lose some of their filtering capability.
Accordingly, a problem with the prior art techniques is that arrangements comprising electrostatic filters lose capacity due to that the electrostatic charge of the filters decrease during operation.